An example of conventional actuator device that employs a piezoelectric element is described in Patent Document 1. The actuator device described in Patent Document 1 is provided with two lens groups in a zoom lens and respective driving means for driving them so as to effectively make use of the optical axis direction region that is required for mutual movement of the lens groups, the region not having been made use of.
This configuration makes it possible to reduce the size of the lens drive mechanism along the optical axis directions. Specifically, it is described that, by disposing the two drive means for the lens groups opposite to each other so as to overlap in the optical axis direction, further size reduction of the lens drive mechanism becomes possible (see Patent Document 1).
Patent Document 2 describes an actuator device for abruptly expanding and contracting a piezoelectric element by supplying sawtooth wave drive voltage to the piezoelectric element.
FIG. 16 illustrates a schematic structural view of one example of the actuator device that uses a piezoelectric element. An actuator device 100 as illustrated in FIG. 16 has a piezoelectric element 103, a weight 102 provided on one end of the piezoelectric element 103, and a drive shaft 104 provided on the other end of the piezoelectric element 103. These materials are adhesive-bonded to one another by an adhesive or the like.
The drive shaft 104 is provided with an engaging member (moving member) such as to catch and hold the shaft from both sides of the shaft. Here, the engaging member comprises a slider 107, a holder 105, and a spring 106 as an urging means for causing the slider 107 and the holder 105 to friction-engage with the drive shaft.
The piezoelectric element 103 is constructed by stacking (piling up) piezoelectric elements of lead zirconate titanate (hereinafter “PZT”). An example of PZT is a sheet-shaped one having a thickness of about 50 [μm] and a diameter Φ of 1.8 [mm]. An electrode of silver, palladium, or the like is provided for imparting an electric field to both sides thereof. When 60 layers of this sheet are stacked, the total thickness becomes about 3.5 [mm].
The weight 102 is made of a metal having a diameter Φ 3 [mm] and a length of 1 [mm], such as tungsten, which has a high density. The drive shaft 104 is made of long carbon fibers having a diameter Φ of 1 [mm] and a length of 4 [mm] that are aligned in an axis direction and fixed by an epoxy resin as a binder.
The weight 2 configured in this way has a substantially symmetrical shape, as illustrated in FIG. 16, and its center of gravity G2 lies at its center part. The drive shaft 104 also has a substantially symmetrical shape and its center of gravity G1 lies at its center part.
By supplying a sawtooth wave drive voltage to this piezoelectric element 103, the drive shaft attached to the piezoelectric element expands and contracts violently, and the moving member friction-retained to the drive shaft repeats “sliding” and “non-sliding” and becomes movable (see Patent Document 2).    Patent Document 1: JP-A-2002-131611 (pp. 3-4, FIG. 6)    Patent Document 2: JP-A-2002-218773 (pp. 3-4, FIGS. 1 and 3)
As an application example of the actuator device 100, FIG. 17 shows a schematic structural view of an image pickup unit 120 that employs the actuator device 100. As illustrated in FIG. 17, the image pickup unit 120 has an aspherical lens 121, an optical filter 123, and a semiconductor image pickup element 124, which are disposed along the optical axis in a housing 125. The lens 121 is retained by a lens-barrel 122, and an engaging member 127 that engages with the drive shaft 104 of the actuator device 100 is provided in a portion of the lens-barrel 122.
The lens-barrel 122 is allowed to be movable by the actuator device 100, in which the drive shaft 104 is disposed parallel to the optical axis. This permits the lens-barrel 122 to move in the optical axis direction so that the subject can be focused on the image pickup plane of the image pickup element 124 precisely with respect to the distance to the subject.